Change the image of the UV/image editor with python for 2.8
Try this:

import bpy

for area in bpy.context.screen.areas:
    if area.type == 'IMAGE_EDITOR':
        area.spaces.active.image = bpy.data.images['Your_image_name']


Deactivate image in Image Editor

for area in bpy.context.screen.areas:
    if area.type == 'IMAGE_EDITOR':
        area.spaces.active.image = None